I agree with K-mike. It is almost certainly the aerator. You can try to grip it by hand using one of the sticky rubber pads that you use to open jars in the kitchen. If that doesn't work, grip the rubber pad with a pair of slip-joint (aka water pump) pliers as gently as possible, but do not apply the pliers directly to the aerator ring! Try to ease it back and forth just a tiny bit until it comes loose.
Remember the advice about which direction is which. It is also helpful to put a mirror under the spout to confirm what the aerator ring looks like, and exactly where you should be gripping. Some spouts have an unusual shape, and when you think you are gripping the aerator, you are actually gripping something else that isn't supposed to turn.
It may help to de-pressurize the plumbing, since pressure will tend to jam a clogged aerator.
Bill
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